86 Prof. Clausius on the Internal Work 



of which we can define the effect of heat as simply tending to in- 

 crease the disgregation. The way in which a definite measure of 

 this magnitude can be arrived at will appear from the sequel. 



In the case last mentioned, an alteration in the arrangement 

 of the electricity takes place, an alteration which can be repre- 

 sented and taken into calculation in a way corresponding to the 

 alteration of the position of the molecules, and which, when it 

 occurs, we will consider as always included in the general expres- 

 sion alteration of arrangement} or alteration of disgregation. 



It is evident that each of the kinds of alteration that have been 

 named may also take place in the reverse sense, if the effect of 

 the opposing forces is greater than that of the heat. We will 

 assume as likewise self-evident that, for the production of work, 

 a corresponding quantity of heat must always be expended, and 

 conversely, that, by the consumption of work, an equivalent quan- 

 tity of heat must be produced. 



§ 3. If we now consider more closely the various cases which 

 occur in relation to the forces which are operative in each of 

 them, the case of the expansion of a permanent gas presents 

 itself as particularly simple. We may conclude from certain pro- 

 perties of the gases that the mutual attraction of their molecules 

 at their mean distances is very small, and therefore that only a 

 very slight resistance is offered to the expansion of a gas, so that 

 the resistance of the sides of the containing vessel must maintain 

 equilibrium with almost the whole effect of the heat. Accord- 

 ingly the externally sensible pressure of a gas forms an approxi- 

 mate measure of the separative force of the heat contained in the 

 gas; and hence, according to the foregoing proposition, this 

 pressure must be nearly proportional to the absolute tempera- 

 ture. The internal probability of the truth of this result is indeed 

 so great, that many physicists since Gay-Lussac and Dalton have 

 without hesitation presupposed this proportionality, and have 

 employed it for calculating the absolute temperature. 



In the above-mentioned case of thermo-electric action, the 

 force which exerts an action contrary to that of the heat is like- 

 wise simple and easily determined. For at the point of contact 

 of two heterogeneous substances, such a quantity of electricity 

 is driven from the one to the other by the action of the heat, 

 that the opposing force resulting from the electric tension suffices 

 to hold the force exerted by the heat in equilibrium. Now in a 

 former memoir " On the application of the Mechanical Theory of 

 Heat to the Phenomena of Thermal Electricity*," I have shown 

 that,in so far as changesinthe arrangementof the molecules arenot 

 necessarily produced at the same time by changes of temperature^ 

 the difference of tension produced by heat must be proportional 

 * Poggendorff's Annalen, vol. xc. p. 513. 



